1. Field of Invention
The present invention relates to the field of wind power generation technology. More particularly, the present invention relates to an excitation control circuit and an electrically excited wind power system including the excitation control circuit.
2. Description of Related Art
Currently, with the increasingly serious energy crisis and environmental problems, countries all over the world are vigorously developing renewable energy business, such as wind power generation and solar electrical energy generation. Taking the wind power generation as an example, from a stall-regulated wind power system to a variable-speed-and-constant-frequency (VSCF) wind power system, and from a wind power system with a gearbox to a direct-drive wind power system without the gearbox, the installed capacity of wind power generation in China grows rapidly. Specifically, with a continuously increasing unit capacity of a current wind power generator, the VSCF technology gradually plays a dominant role. In a current megawatt wind turbine generator, the gearbox is a component with a relatively high damage rate, so that the direct-drive wind power system without the gearbox receives more emphasis and development due to its advantages of low noise, long turbine operation time, and low cost for operation and maintenance.
In general, a direct-drive wind power system mainly has two manners including permanent magnetism and electrical excitation. However, currently, the rare earth material is expensive, which directly increases the manufacturing cost of a permanent-magnet wind power system. Comparatively, the electrically excited wind power system gradually becomes the main trend in research and development for technical professionals.
In the prior art, the structure of an electrically excited wind power system is to rely on the DC bus of a single set of back-to-back converters (formed from a generator-side converter and a grid-side converter) to provide the input voltage of a DC-DC converter, and to output an appropriate DC voltage by the DC-DC converter through a bucking process, so as to supply power to the excitation device. However, when the DC-DC converter is out of order, the wind power generator is very likely to be suddenly de-excited to cause the torque to disappear, such that the converter cannot provide a brake torque to the wind power generator, thus damaging components in the wind power generation system. In another structure of an electrically excited wind power system, through the AC voltage of the power grid, a main excitation switch, an industrial frequency transformer and a controlled rectifier bridge are sequentially arranged to output a rectified DC voltage which is then directly provided to a DC-DC converter, and thereafter an appropriate DC voltage is outputted by the DC-DC converter through a bucking process so as to supply power to the excitation device. However, the industrial frequency transformer has a big volume and high cost. More seriously, when the voltage of the power grid has voltage drop or power down, the DC input voltage of the DC-DC converter disappears, and the wind power generator is suddenly de-excited, thus causing an abrupt torque change.
In view of the foregoing, there more or less exists the condition that the wind power generator is suddenly de-excited to cause the torque to disappear in the conventional electrically excited wind power system, which significantly reduces the reliability of electrical excitation and the operation stability of the system. In view of this, those in this industry are endeavoring to find ways to design a reliable excitation control circuit for improving the working stability of the excitation device and ensuring that the magnetic filed not to disappear suddenly.